Presently, many document scanners and digital copiers are calibrated using the following calibration strategy, yi=mixi+bi. For every i pixels there is a gain factor (mi) and offset factor (bi) applied to the image sensor response (xi) to obtain the desired image sensor response (yi). The image sensor response and desired image sensor response can be in voltage or A/D output levels, depending on whether the system calibration is done in analog or digital circuitry. The values of bi are set so that all pixels give the same desired minimum value of y with no light signal, and mi values are set so that all pixels give the same maximum value of y with the maximum possible light signal. These values can be determined with a calibration scan to a reference white strip and a dark (no light) capture. The values of mi and bi are stored in memory and used during real-time correction of image sensor output.
FIG. 1 shows the internal parts of a typical document handler 1 for a document scanner in a digital copier having a controlled velocity transport (CVT) roll 2. The roll 2 has a gap 3 in it to allow space for paper position sensors (not shown). This gap 3 presents a non-uniform background for documents and the potential for variation in scanner response for lightweight originals, translucent and/or transparent with high document show-through. Another potential non-uniform background area is drive roll contamination 4, e.g., toner or paper dust, at the locations of contact with the idler rolls 5 or at other locations on the roll 2.
The above calibration strategy does not compensate for the non-uniform background. Therefore, for light weight originals and/or for certain machine setups (copy darker/document type), the signature of the drive roll gap may be captured by the scanner and may create an objectionable band down the center of the resulting document's printed copy.
FIG. 2 show an original document 7 to be scanned. The drive roll 2 is white and provides a uniform background to documents that may be somewhat translucent. The gap 3 in the drive roll 2 causes the background characteristics to be darker in this region. In this gap region, the background is essentially black. Therefore, there exists a discontinuity of backing reflectance, going from the white roll to the dark gap area. The dark gap area ‘show-through’ is imaged by the scanner and appears darker than the real document. This results in the streak-like gap defect 9 on the resulting scanned copy 8, as shown in FIG. 3. The machine image processing, in most cases, will mask this problem, through the use of a Tone Reproduction Curve which suppresses the highlight details. However, this is not a robust solution as it fails under certain use conditions typically with light weight and translucent originals (user interface (UI) selections) and produces darker images, which may bring out the darkness of the gap. It may also occur when the user changes the settings.
FIG. 4 illustrates the Tone Reproduction Curve (TRC) response profiles for the current-compromised system TRC and the desired system TRC for a typical scanner. In general, the default settings for the tonal reproduction of the digital copier system have been optimized to eliminate the ‘show-through’ problem. This is accomplished by designing the TRC, such that it suppresses the highlight details for high L* values. Any light shades of gray or high luminance colors will not be reproduced because they are eliminated from the printed image by the image processing system. Therefore, the default settings do not provide a faithful reproduction of the original document reflectance. In order to restore these missing details, the user must select a different setting (UI adjustment) until the light gray or color information is restored. However, in doing so, they may also encounter the presence of the show-through of the gap in the CVT backing rolls, depending on the transmission characteristics of the input document substrate.
Other alternative solutions, such as changing the color of the drive roll from white to gray, have been considered but do not eliminate the problem completely. This approach still has a differential backing reflectance; gray on the roll and black in the gap. This approach introduces a new artifact in the reproduction of the gray backing in areas where the document substrate is missing. This includes 3-hole punched paper, torn corners, or mis-registration due to mechanical variations.
Thus, there is a need for an improved method for removing show-through defects in scanned images.